1. Field of the Invention
The invention relates generally to radio antennas and more specifically to an antenna combination for receiving a Global Positioning System (GPS) signal and a radio signal having differential GPS (DGPS) correction information.
2. Description of the Prior Art
Global Positioning System (GPS) receivers are commonly used for navigation and precise positioning. The GPS receivers include a GPS antenna to receive a GPS signal having location determination information from one or more GPS satellites and a receiver unit to process the information and provide a GPS-derived location to a user. The GPS signal includes two codes--a P-Y code that is only available to agencies authorized by the United States Military Services and a C/A code that is available to all users. The inherent accuracy of the C/A code is about twenty meters. However, at the present time the United States Government degrades the location determination information in the C/A code with selective availability (SA) so that the accuracy for most users is currently about one hundred meters. Unfortunately, twenty to one hundred meters location accuracy is insufficient for precise positioning applications. Such applications require an accuracy in a range of less than a centimeter for boundary surveying to a few meters for geographical information systems.
Several differential GPS (DGPS) systems exist or have been proposed for improving the accuracy of the GPS-derived location by using differential GPS (DGPS) corrections. These systems include a reference GPS receiver located at a known location to compute the DGPS corrections by comparing the GPS location determination information to information representing the known location; and a differential-ready GPS receiver system, at a site whose location is to be determined, to apply the DGPS corrections to the GPS location determination information and provide a differentially corrected GPS (DGPS)-derived location. When the reference and the differential-ready GPS receivers are located within a few hundred kilometers and receive some of the same GPS satellite signals at the same time, the DGPS corrections can be used to correct the inherent or the SA-degraded accuracy to a range of less than a meter to a few meters.
The DGPS corrections are available in FM radio signals broadcasted from various stations at frequencies in a range of 88 to 108 MHz. In order to provide the DGPS location in real time, the differential-ready GPS receiver includes a radio antenna to receive the FM radio signal and an FM DGPS radio receiver to demodulate the signal and provide the DGPS corrections. For convenience in mobile applications, for low cost, and for a clear reception path to receive the FM radio signal, it is desirable for the radio antenna to be packaged with the GPS antenna as a GPS/radio antenna combination and to use a single cable to connect to the receiver unit. However, existing radio antennas for receiving FM radio signals are packaged separately and use a separate cable or are integrated into an FM receiver unit.
A difficulty in producing a GPS/radio antenna combination is that the electronics and mechanical structure for receiving the FM radio signal tend to interfere with the performance of the GPS antenna for receiving the GPS signal. One solution to this difficulty is disclosed in U.S. patent application Ser. No. 08/354,617 for a "Dual Mode Vertical Antenna" by Rodal. However, the vertical GPS antenna disclosed by Rodal is not circularly polarized. Without circular polarization, the GPS antenna is more susceptible to multipath GPS signals that reduce the accuracy of the differentially-corrected GPS (DGPS) location.
There is a need for a GPS/radio antenna in a single package using a single output cable that combines a GPS antenna having circular polarization for receiving GPS satellite signals and a radio antenna for receiving a radio signal having DGPS corrections. There is a further need for a GPS receiver system, using such a GPS/radio antenna, for providing a differentially-corrected GPS (DGPS) location.